Roseanne's & Bebe's are by far the best trainers to get early ... and they are needed in nearly every deck.
It's also usually a lot cheaper to buy a theme deck or 2 and then aim for indivicual cards you need in local card shops or on eBay than it is to buy random stuff, though I'll grant that random is sometimes more fun.
IMHO best idea is to have an initial plan that fits your budget and then go for it.
You can go to SteveP's decklist program (click the Sample Decks) to see what is in each deck:
http://pokegym.net/decklist/
Edit:
Here is a plan that I might follow in your shoes:
Step 1 - buy Drill Point & Overflow then consolidate the two decks (roughly $26 to $28 - why these two? Because I can build something with reasonable draw power with all the trainrs here & the Pokemon in the two sets can work together ... Rebellion would be be my second choice to Drill Point because Ralts is a GREAT starter & I really like the trainers there too)
Pokemon (22)
4-2-1 Flygon
4-2-1 Swampert
2-1 Hippowdon [To be changed later for stuff like uxie, claydol, etc.]
3-2 Floatzel [To be changed later for stuff like uxie, claydol, etc.]
Trainers (20)
1 Aaron's Collection [To be changed later for Night Maintance or Palmer's]
2 Cynthia's Feelings
2 Bebe's Search
2 Felicity's Drawing
2 Roseanne's Reserach
2 Volkner's Philosophy
4 Potion [To be changed later for Rare Candy]
1 Night Teleporter
4 Switch [To be changed later to Warp Points or other trainers - but keep at least 2 or 3 Switch or Warp]
Energy (18)
9 Fighting
9 Water
[you will not need this much energy later on - trade some of these for Call energy - you may want to look at the Flygon decks in Deck help and start building towards one of them]
Step 2 - I would try to buy the following cards
* Call Energy
* Rare Candy
* Night Maintenance or Palmers
* Flygon (but only if it's cheap)
I'll assume that you get 2 Call Energy and a Night Maintenance and then make the following changes:
Pokemon (22)
4-2-1 Flygon
4-2-1 Swampert
2-1 Hippowdon [To be changed later]
3-2 Floatzel [To be changed later]
Trainers (20)
2 Cynthia's Feelings
2 Bebe's Search
2 Felicity's Drawing
2 Roseanne's Reserach
2 Volkner's Philosophy
4 Potion [To be changed later]
1 Night Maintenance
1 Night Teleporter
4 Switch [To be changed later]
Energy (18)
8 Fighting
8 Water
2 Call Energy
Step 3 - find my local league and play as many games as I can so I can earn Uxies now and possibly even better stuff like Claydol later. You can also trade for stuff here and see what other people are playing. Testing your deck will show you what you like and don't like.
Step 4 - spend more money over time to customize the deck. Possibly getting more Bebe's, Roseanne's, Flygons, etc. You *might* want to buy another Drill Point deck since you get bebe's and flygons from it.